In modern computers or other electronic devices, increasing the size of memory usually leads to enhanced performance. The memory of a computer or other electronic device is typically volatile storage (implemented in the form of dynamic or static random access memories) provided between a processor and persistent storage (typically implemented with disk-based storage devices).
The memory is implemented with memory devices having faster access speeds than persistent storage. The memory devices are usually provided in memory modules, with each memory module having plural memory devices.
Examples of memory modules include single in-line memory modules (SIMMs) or dual in-line memory modules (DIMMs). A DIMM may have a data path that is larger than a data path of a SIMM. A memory module, such as a SIMM or DIMM, typically has a support substrate on which memory devices can be mounted on both sides of the support substrate. The electrical contact pins of the memory module are also provided on both sides of the support substrate. The electrical contact pins are connected to corresponding contact points in a connector of a system board. The electrical contact pins of the memory module, when inserted in a system board connector, connect power and ground voltages, address signals, data signals, and control signals to the memory devices on the memory module.
Various issues are associated with conventional memory modules. One is the issue of noise on power lines on the memory module. The power lines connect power voltages from the power pins to the memory devices. Conventional memory modules usually employ two different sets of power pins, with one set of power pins used to power the core circuitry of each memory device, and another set of power pins used for powering the input/output (I/O) circuitry of each memory device. The core circuitry of a memory device refers to the memory cells and associated peripheral circuitry around the memory cells of the memory device. The I/O circuitry refers to the input/output buffers and drivers of the memory device. The presence of the two sets of power pins means that decoupling capacitors on the memory module cannot be shared for reducing noise. The inability to share decoupling capacitors makes the memory module layout more complex and inefficient.
Another issue associated with conventional memory modules is that power pins of the memory module may be spaced apart from ground pins by intervening signal pins. This spaced apart relationship between power and ground pins increases the impedance between the power and ground pins, which leads to increased noise on a memory module. Also, in some conventional memory modules, some signal pins use a power pin (instead of a ground pin) as a reference, which also leads to increased noise, if proper decoupling is not used to tie the planes together to form a low impedance path between the planes at all relevant frequencies.
A further issue of conventional memory modules is reduced reliability in light of the large number of pins that are provided on the memory module. For example, the Joint Electron Device Engineering Council (JEDEC) has defined a pin arrangement of a DIMM with 240 pins. With such a large number of pins, the likelihood that any one DIMM pin may experience poor electrical connection with a corresponding contact point of a system board connector is increased. If the pin with a poor electrical connection is an address pin or control pin, then one or more of the memory devices on the DIMM may not function properly.